Electronic Couplings in Dimers

Each WCP dimer model consists of two purine bases (G and A) and two pyrimidine bases (C and T). According to the calculations, the two highest-lying orbitals HOMO and HOMO-1 of each duplex are mainly locahzed on the purine nucleobases, whereas the two occupied MOs following at lower energies, HOMO-2 and HOMO-3, are locahzed on pyrimidine nucleobases. Therefore, the purine-purine electronic coupling provides the dominant contribution to the hole transfer matrix elements, irrespective whether the bases belong to the same or to opposite strands. 

Hole transfer can exhibit a pronounced directional asymmetry. For instance, the couphng matrix element 1 (G-A) in [(GC),(AT)] is about five times larger than 1 (A-G) in [(AT),(GC)]. In both systems, G and A are in the same strand with the orientations 5 -G-A-3 and 3 -A-G-5, respectively. On the other hand, very similar electronic couphngs G/A and G A, with distinct interstrand orientation of G and A, are calculated for the systems [(GC),(TA)] and [(CG),(AT)], respectively. 

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